This invention relates to methods of manufacturing metal base transistors and permeable base transistors. Specifically, this invention relates to a method for making dielectrically isolated metal base transistors and permeable base transistors.
Recently, because of their superior high frequency and high current gain characteristics, interest has been generated in metal base transistors, which are transistors in which the base of the transistor is a metal, or more particularly a metal silicide such as cobalt silicide. Furthermore, a variant of the metal base transistor, known as the permeable base transistor is also of interest as it also has favorable operating characteristics. In a permeable base transistor the electrons will tunnel directly through so-called "pin holes" in the metal base. Both of these transistors have high gain and fast response characteristics which makes them particularly suitable for high frequency applications.
For high frequency operation each of the metal base or permeable base transistors must be dielectrically isolated from the others. The most common method for providing dielectric isolation to these transistors has been by p-n doping of the underlying substrate. However, the individual transistor and its accompanying doping region has a small, but far from insignificant, capacitance. Furthermore, the effect of this residual capacitance becomes significant at higher and higher frequencies. This capacitance will limit the effective speed of the device. The present invention is directed towards a method of making metal base transistors and permeable base transistors which are completely dielectrically isolated.